SCEV: Compress disposition pairs.

Composing DenseMaps and SmallVectors is still somewhat suboptimal,
but this at least halves the size of the vector elements. NFC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228497 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed